For several decades, seismic prospecting for petroleum has involved the creation of acoustic disturbances above, upon, or just below the surface of the earth, using explosives, air guns, or large mechanical vibrators. Resulting acoustic waves propagate downwardly in the earth, and are partially reflected back toward the surface when acoustic impedance changes within the earth are encountered. A change from one rock type to another, for example, may be accompanied by an acoustic impedance change, so that the reflectivity of a particular layer depends on the velocity and density content between that layer and the layer which overlies it, say according to the formula ##EQU1## where AR is the amplitude from the reflected signal and Ai is the amplitude of the incident signal; V.sub.1 is the compressional velocity of the wave in the overlying medium 1; V.sub.2 is the compressional velocity in the medium layer below the contact line; d.sub.1 is the density of the overlying medium 1; and d.sub.2 is the density of the underlying medium.
Today's seismic interpretors have made good use of ultra-high amplitude anomalies in seismic traces to infer the presence of natural gas in situ. So-called "bright-spot" analysis has been used to good advantage to indicate several large gas reservoirs in the world, especially in the Gulf Coast of the United States. Such analysis is now rather common in the oil industry, but it is not without its critics, especially in the area of predicting gas saturation based on the characteristics of the amplitude anomalies of the traces alone.
The present invention improves the ability of the seismologist to correctly interpret gas as well as gas/oil content of a formation normalized to (and comparable with) a series of patterned acoustic characteristics associated with zones of similar mineralogy and determinable gas or gas/oil saturations.